Good old water, aka H2O. The most common skincare ingredient of all. You can usually find it right in the very first spot of the ingredient list, meaning it’s the biggest thing out of all the stuff that makes up the product.

It’s mainly a solvent for ingredients that do not like to dissolve in oils but rather in water.

There is not much research out there about malic acid itself. We could find one comparative study, that compared the effectiveness of glycolic acid, lactic acid, citric acid and malic acid. The first two ones were the winners while malic and citric acid were less effective.

The thing that you put on your salad with some olive oil. But in a more scientific sense vinegar is usually a 5% solution of acetic acid. It's used in cosmetic products to adjust pH. Larg amounts of vinegar on the skin could be irritating and drying.

Though its name says acid, it's not really an exfoliant. It's a plant extract with some antioxidant properties. Its main thing in cosmetic products is to neutralize the metal ions in the formula (that usually get into there from water) that would otherwise cause some not so nice changes. It's a natural alternative to sometimes bad-mouthed chelating agents, EDTAs.

There is also some research showing that citric acid with regular use (think three months and 20% concentration) can help sun-damaged skin, increase skin thickness and some nice hydrating things called glycosaminoglycans in the skin.

But according to a comparative study done in 1995, citric acid has less skin improving magic properties than glycolic or lactic acid. Probably that’s why citric acid is usually not used as an exfoliant but more as a helper ingredient in small amounts to adjust the pH of a formulation.

Horseradish seems to be a rarely used plant extract with little info about what it's doing in a cosmetic product. Some sources say that it's purifying and stimulating, while others say that it's antibacterial and anti-inflammatory. Paula Begoun thinks that horseradish is an irritant, but she does not cite any sources.

Burdock root is a goodie plant extract that has anti-inflammatory and antioxidant properties. Also, it has some anti-fungal and seborrhea regulation magic powers that make the ingredient especially useful for greasy and flaky skin and scalp.

The extract of cayenne pepper that contains capsaicin. According to manufacturer info it can stimulate the skin blood circulation that can be useful for anti-cellulite or massage products. Be careful with it if your skin is sensitive because it's also a skin irritant.

A nice yellow flower living in themountains. It has been used as a herbal medicine for centuries, though its effect on skin is rather questionable. It's most famously used to treat bruisings, but there are some studies that show that it's not better than placebo (source: wikipedia). Also, some consider it to be anti-inflammatory, while other research shows that it can cause skin irritation.

A yellowish element that smells of rotten eggs and it might be familiar to you from the periodic table (has the symbol S in there). It has a long history of medicinal use thanks to its antifungal, antibacterial and keratolytic activity. It used to be a very common ingredient in the treatment of inflammation-related skin diseases such as acne, rosacea and seborrheic dermatitis, however, due to its malodorousness, it is less popular nowadays.

Sulfur's precise mechanism of action is not known, but we do know that its effectiveness depends on its direct interaction with the skin surface, meaning the smaller the particle size, the better the effect. The United States Pharmacopeia lists two types of sulfur, sublimed and precipitated. The latter one has a smaller particle size and counts as a superior version.

Sulfur is also a team-player and works well when combined with other anti-acne agents, such as salicylic acid or sodium sulfacetamide. In fact, the combination of 10% sodium sulfacetamide and 5% sulfur is the active ingredient duo of several Rx-only anti-acne products.

As for the disadvantages, there is the rotten egg smell. Also in higher concentrations, it might cause a mild burning sensation and dry skin. If you are new to sulfur, patch testing it first is a good idea.

The sodium salt of salicylic acid. CosIng (the official EU cosmetic ingredient database) says that it's a preservative and helps to make the product taste bad (called denaturant), while some manufacturer claims that it has exfoliating properties and is antimicrobial. It's good to know, that the salt of an exfoliant is kind of a neutralised form, so if you want to go for exfoliation stick to the pure acid.

Serine is an amino acid that most often comes to the formula as part of a moisturizing complex. It's a non-essential amino acid (meaning that our body can synthesize it) and serves as a water-binding ingredient.

In general, amino acids are great skincare ingredients that play an important role in proper skin hydration but there is not much info out there about what specifically serine can do for the skin.

Yes, it's the thing that can be found naturally in pee. But it is also a natural moisturizing factor (about 7% of skin NMF is urea) found in our skin cells where it helps the skin to cling onto water and stay nicely hydrated.

Don't worry, the urea version used in skincare is not extracted from pee, but it is synthetic and it's a very effective moisturizer. Plus, unlike a bunch of other water-binding ingredients, it is also nice to use as it's non-tacky and non-greasy.